1. Field of the Invention
The present invention generally relates to a Y/C separation circuit. More specifically, the present invention relates to a comb filter type Y/C separation circuit in which a luminance signal and a chrominance signal are separated from each other on the basis of an input composite video signal and a signal outputted from a glass delay line having a delay time of one horizontal scanning line (1H).
2. Description of the Related Art
FIG. 1 shows one example of such a kind of comb filter type Y/C separate circuit 1. The Y/C separation circuit 1 separates a luminance signal and a chrominance signal from each other by utilizing a fact that a phase of the chrominance signal is inverted every other line. To this end, a delay time of a glass delay line 2 must be exactly 1H. However, the glass delay line 2 generally has a structure in which a pair of supersonic transducers are provided at diagonal corners of a rectangular glass substrate and an electrical signal inputted to one supersonic transducer and transduced into a supersonic signal which is propagated on a surface of the glass substrate to the other supersonic transducer, and an electrical signal being delayed by 1H is withdrawn from the other supersonic transducer. Therefore, due to a working condition of the surface of the glass substrate, dispersion occurs in a delay time. Therefore, as shown in FIG. 1, the glass delay line 2 is terminated by a variable inductance coil L and, by adjusting an inductance value of the variable inductance coil L, such dispersion of the delay time in the glass delay line 2 is corrected or compensated.
In addition, in order to place an amplitude of a composite video signal inputted from an input terminal 3 close to an amplitude of a video signal outputted from the glass delay line 2, by performing a gain adjustment of a variable amplifier 4, dispersion of an insertion loss is also corrected.
Furthermore, FIG. 2 shows a frequency-gain characteristic of a luminance signal at respective points a-d in FIG. 1. A composite video signal as shown in FIG. 2(A) is inputted at the point (a). As seen from FIG. 2(B), though a luminance signal having a frequency component in the vicinity of a color sub-carrier signal is outputted from the glass delay line 2 at the point (b) and thus a chrominance signal is attenuated by the glass delay line 2, as shown in FIG. (C), the luminance signal outputted at the point (c) from an adding circuit 5 includes a chrominance signal component. Therefore, in order to suppress a level of the chrominance signal component, a trap circuit 6 is provided at the output of the adding circuit 5 so as to reduce the chrominance signal component in the luminance signal so that deterioration of a video quality can be prevented. In addition, the chrominance signal is outputted from a subtracting circuit 7.
However, in the conventional Y/C separation circuit 1 shown in FIG. 1, since the variable inductance coil L and the variable amplifier 4 are respectively adjusted manually, not only it takes a long time for adjustment but also dispersion occurs in performance for each product.